Semiconductor device and method for producing semiconductor device

ABSTRACT

Proton irradiation is performed a plurality of times from rear surface of an n-type semiconductor substrate, which is an n −  drift layer, forming an n-type FS layer having lower resistance than the n-type semiconductor substrate in the rear surface of the n −  drift layer. When the proton irradiation is performed a plurality of times, the next proton irradiation is performed to as to compensate for a reduction in mobility due to disorder which remains after the previous proton irradiation. In this case, the second or subsequent proton irradiation is performed at the position of the disorder which is formed by the previous proton irradiation. In this way, even after proton irradiation and a heat treatment, the disorder is reduced and it is possible to prevent deterioration of characteristics, such as increase in leakage current. It is possible to form an n-type FS layer including a high-concentration hydrogen-related donor layer.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates to a semiconductor device and a method forproducing a semiconductor device.

B. Description of the Related Art

A power conversion apparatus, such as a converter-inverter systemessential for the control of a rotary motor or a servo-motor, has beenknown. In order to improve the efficiency of the power conversionapparatus and to reduce the power consumption thereof, there is a strongdemand for a technique capable of reducing the loss of a semiconductordevice, such as a power diode or an insulated gate bipolar transistor(IGBT) provided in the power conversion apparatus.

As one of the methods for meeting the demand for the technique capableof reducing the loss, for a diode or an IGBT, a field stop (FS) layerstructure has been known in which a drift layer, which is a thick layerwith the highest resistance, among semiconductor layers forming anelement structure is thinned to reduce a voltage drop due to anon-current, thereby reducing on-loss. In the FS layer structure, an FSlayer which has a higher impurity concentration than the drift layer andhas the same conduction type as the drift layer is provided at aposition that is away from the breakdown voltage main junction of thedrift layer in the drift layer. The provision of the FS layer makes itpossible to suppress the spreading of a depletion layer which is spreadfrom the breakdown voltage main junction in the high-resistance driftlayer when the device is turned off. Therefore, it is possible toprevent punch-through even when the drift layer is thin.

In the manufacture (production) of the power device, a wafer(hereinafter, referred to as an FZ wafer) which is cut out from an ingotproduced by a floating zone (FZ) method is used in order to reducecosts. The FZ wafer with a thickness of 600 μm or more is put into amanufacturing process in order to reduce the breaking of the wafer.Finally, the FZ wafer is ground to a thickness required for the designedbreakdown voltage during the manufacturing process in order to reduceon-loss. In particular, in a MOS (metal-oxide film-semiconductor)device, such as an IGBT, after a MOS gate structure, a circumferentialjunction edge termination structure, and a metal electrode film areformed on the front surface of the FZ wafer, a grinding process forthinning the FZ wafer is performed for the rear surface of the FZ wafer.Then, after the rear surface of the FZ wafer is ground to reduce thethickness of the wafer, an FS layer or a collector layer is formed onthe ground rear surface of the FZ wafer. Therefore, in the methodaccording to the related art, there are restrictions that the FS layeris formed under the conditions that have no adverse effect on thesemiconductor function layers provided on the front surface side of theFZ wafer. Therefore, it is not easy to form the FS layer. In general,the FS layer is formed with, for example, an n-type impurity elementwith a large diffusion coefficient. In some cases, in addition to an FZwafer which is made of polysilicon with high crystal purity, an FZ wafermade of a CZ wafer or a CZ wafer with high resistivity is used.

In recent years, a method has been developed which forms the FS layerusing a process of generating donors using proton irradiation. In themethod for forming the FS layer using proton irradiation, a heattreatment is performed to recover the crystal defects which aregenerated in an FZ bulk wafer by irradiation with proton ions (H+) andprotons in the vicinity of the average range Rp of protons in the FZbulk wafer are changed into donors to form an n-type region with highconcentration.

In some cases, when the n-type region with high concentration is formedby proton irradiation, the mobility of electrons/holes is reduced at theirradiation position of the proton, which is described in the patentliterature (for example, see the following Patent Literature 1). Inaddition, proton irradiation conditions for forming a blocking zone (FSlayer) and the preferred heat treatment conditions after protonirradiation have been proposed when the n-type region with highconcentration is formed by proton irradiation (for example, see thefollowing Patent Literature 3 to the following Patent Literature 7).Unlike other ions, protons are combined with the crystal defects in thesemiconductor layer to recover carrier concentration. As theconcentration of the crystal defects generated in the semiconductorlayer during proton irradiation increases, higher carrier concentrationis obtained, which is described in the patent literature (for example,see the following Patent Literature 2).

The following Patent Literature 1 discloses a region in which themobility of electrons/holes is reduced due to proton irradiation.Specifically, it has been reported that the mobility of carriers isreduced by a high-concentration crystal defect layer which is formed inthe vicinity of the rear surface of the wafer by proton irradiation. Thefollowing Patent Literature 2 discloses a structure in which, when thecrystal defects generated by proton irradiation are recovered by a heattreatment, the remaining amount of crystal defects is so large that adonor layer is not removed by protons. The general impurity atoms, suchas phosphorous (P) atoms or arsenic (As) atoms which are present at thelattice positions of silicon (Si), exchange an outermost electron. Incontrast, in the above description, a donor (hereinafter, referred to asa hydrogen-related donor) caused by hydrogen (H) supplies an electronfrom a composite defect of a plurality of lattice defects (for example,divacancies) which are formed in silicon by proton irradiation and theradiated hydrogen atom.

CITATION LIST

Patent Literature 1: US 2005/0116249

Patent Literature 2: JP 2006-344977 A

Patent Literature 3: US 2006/0081923

Patent Literature 4: JP 2003-533047 W

Patent Literature 5: US 2009/0186462

Patent Literature 6: US 2008/0001257

Patent Literature 7: US 2007/0120170

SUMMARY OF THE INVENTION

However, as described above, when the hydrogen-related donor is formedwith a higher concentration than the impurity concentration of thesemiconductor substrate (FZ wafer) by proton irradiation, a large amountof disorder (a state in which lattice defect density is high and themoving distance of atoms from a crystal position is large and which isclose to an amorphous state) is introduced into the semiconductorsubstrate by the proton irradiation. As a result, the mobility ofcarriers is greatly reduced from an ideal value in a crystal. In a casein which a device is produced in this state, when a depletion layerwhich is spread at the time a voltage is applied to the device reachesthe region in which the disorder remains, a large number of carriers aregenerated from the center of the defect and a large leakage currentwhich is beyond the allowable range is generated. In addition, since themobility of carriers is reduced, the on-voltage of the IGBT increasesand conduction loss increases. Since the disorder in the semiconductorsubstrate becomes a recombination center, carrier concentration isreduced and carriers are likely to be depleted when the device is turnedoff, which causes turn-off oscillation.

When the crystal defects in the semiconductor substrate are recovered byan annealing process to remove the disorder in order to solve theabove-mentioned problems, the hydrogen-related donor is also removed bythe annealing process for removing the disorder since it is a compositedefect. As such, there is a trade-off relation between the ensuring ofthe desired hydrogen-related donor concentration and the removal of thedisorder remaining in the semiconductor substrate. In order to improvethe trade-off relation, it is necessary to sufficiently remove thedisorder while leaving the hydrogen-related donor in the semiconductorsubstrate. However, a method for obtaining this state has not beenknown. Therefore, there is an urgent need to develop a new means capableof ensuring the desired hydrogen-related donor concentration even whenthe disorder is sufficiently removed.

In order to obtain good switching characteristics, it is necessary toform the FS layer in the region that is at a depth of 15 μm or more fromthe rear surface of the semiconductor substrate. However, the inventorsfound that, when the average range of proton irradiation was set to 15μm or more in order to form the FS layer in the region that was at adepth of 15 μm or more from the rear surface of the semiconductorsubstrate, a proton passage region which was at a depth of 15 μm fromthe rear surface of the semiconductor substrate was a region in whichcarrier concentration measured by a spread-resistance profiling (SR)method was significantly lower than the doping concentration of thesemiconductor substrate, that is, a disorder region.

FIG. 8 is a characteristic diagram illustrating the relation betweencarrier concentration and the average range of proton irradiation in therelated art. FIG. 8 shows the carrier concentration of a siliconsubstrate measured by the SR method after proton irradiation isperformed for the silicon substrate and then a heat treatment isperformed at a temperature of 350° C. FIG. 8(a) shows a case in whichthe average range of the proton irradiation is 50 μm, FIG. 8(b) shows acase in which the average range of the proton irradiation is 20 μm, andFIG. 8(c) shows a case in which the average range of the protonirradiation is 10 μm. In FIGS. 8(a) to 8(c), the horizontal axis is adistance (depth) from a proton incident surface. In FIG. 8(c), when theaverage range of the proton irradiation is 10 μm, a reduction in carrierconcentration does not appear in, particularly, the proton passageregion. On the other hand, in FIG. 8(b), when the average range of theproton irradiation is 20 μm, the carrier concentration is lower than thesubstrate concentration and a reduction in the carrier concentrationappears. That is, disorder remains in the region. As can be seen fromFIG. 14(a), when the average range of the proton irradiation is 50 μm, areduction in the carrier concentration of the passage region isremarkable and a large amount of disorder remains. As such, when thereis a disorder region in the semiconductor substrate, a leakage currentor conduction loss increases, as described above. Therefore, it isnecessary to remove disorder.

An object of the invention is to provide a semiconductor device whichhas a small amount of disorder and includes a region in whichhydrogen-related donor concentration is high and a method for producingthe semiconductor device, in order to solve the problems of the relatedart.

In order to solve the above-mentioned problems and achieve the object ofthe invention, there is provided a method for producing a semiconductordevice including a breakdown voltage holding pn junction that isprovided in one main surface of an n-type semiconductor substrate and ann-type field stop layer that is provided in the other main surface ofthe n-type semiconductor substrate, has a lower resistance than then-type semiconductor substrate, and suppresses the spreading of adepletion layer from the breakdown voltage holding pn junction. Themethod for producing a semiconductor device has the followingcharacteristics. A proton irradiation step of repeatedly performingproton irradiation a plurality of times from the other main surface ofthe n-type semiconductor substrate to form the n-type field stop layerin the other main surface of the n-type semiconductor substrate isperformed. In the proton irradiation step, whenever the protonirradiation is repeated, the next proton irradiation is performed so asto compensate for a reduction in mobility due to a disorder whichremains in the previous proton irradiation.

In the method for producing a semiconductor device according to theinvention, in the proton irradiation step, an irradiation depth in thenext proton irradiation may be less than an irradiation depth in theprevious proton irradiation.

In the method for producing a semiconductor device according to theinvention, in the proton irradiation step, the second or subsequentproton irradiation may be repeatedly performed on the basis of theposition of the disorder formed by the previous proton irradiation.

In the method for producing a semiconductor device according to theinvention, in the proton irradiation step, the acceleration energy anddose of the proton irradiation may be adjusted such that a portion inwhich impurity concentration is reduced by the disorder formed by theprevious proton irradiation is compensated by a peak of an impurityconcentration distribution formed by the second or subsequent protonirradiation.

In the method for producing a semiconductor device according to theinvention, in the proton irradiation step, a depth at which thereduction in mobility due to the disorder is the maximum after theprevious proton irradiation may be the irradiation depth in the nextproton irradiation.

In the method for producing a semiconductor device according to theinvention, when the common logarithm log(E) of the acceleration energy Eof the proton irradiation is y and the common logarithm log(Rp) of anaverage range Rp of the proton irradiation from the other main surfaceis x, y=−0.0047x⁴+0.0528x³−0.2211x²+0.9923x+5.0474 may be satisfied.

In the method for producing a semiconductor device according to theinvention, the semiconductor device may be a diode or an IGBT.

In order to solve the above-mentioned problems and achieve the object ofthe invention, a semiconductor device according to the invention has thefollowing characteristics. A breakdown voltage holding pn junction isprovided in one main surface of an n-type semiconductor substrate. Ann-type field stop layer which has a lower resistance than the n-typesemiconductor substrate and suppresses the spreading of a depletionlayer from the breakdown voltage holding pn junction is provided in theother main surface of the n-type semiconductor substrate. The n-typefield stop layer has an impurity concentration distribution whichincludes a plurality of impurity concentration peaks at differentpositions in a depth direction of the n-type semiconductor substrate.Among the plurality of impurity concentration peaks, an impurityconcentration peak closest to the one main surface of the n-typesemiconductor substrate is disposed at a depth of 15 μm or more from theother main surface of the n-type semiconductor substrate. A distancebetween the position of the impurity concentration peak in the n-typefield stop layer and the other main surface of the n-type semiconductorsubstrate is equal to or more than half of a distance between theposition of the impurity concentration peak adjacent to the one mainsurface of the n-type semiconductor substrate among the impurityconcentration peaks and the other main surface of the n-typesemiconductor substrate.

In the semiconductor device according to the invention, among theplurality of impurity concentration peaks, an impurity concentrationpeak which is closest to the other main surface of the n-typesemiconductor substrate may be disposed at a depth of 6 μm to 15 μm fromthe other main surface of the n-type semiconductor substrate.

According to the semiconductor device and the method for producing thesemiconductor device of the invention, it is possible to reduce thedegree of disorder generated in the semiconductor substrate after protonirradiation and a heat treatment. In addition, according to thesemiconductor device and the method for producing the semiconductordevice of the invention, it is possible to prevent deterioration ofcharacteristics, such as an increase in leakage current, and form aregion with high hydrogen-related donor concentration.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing advantages and features of the invention will becomeapparent upon reference to the following detailed description and theaccompanying drawings, of which:

FIGS. 1(a) to 1(c) are characteristic diagrams illustrating the impurityconcentration profile of an n-type FS layer in a semiconductor deviceaccording to Embodiment 1.

FIGS. 2(a) and 2(b) are cross-sectional views schematically illustratingthe structure of a general IGBT.

FIG. 3 is a flowchart illustrating the outline of a method for producingthe semiconductor device according to Embodiment 1.

FIG. 4 is a characteristic diagram illustrating another example of theimpurity concentration profile of the n-type FS layer in thesemiconductor device according to Embodiment 1.

FIGS. 5(a) to 5(c) are characteristic diagrams illustrating the impurityconcentration profile of an n-type FS layer in a semiconductor deviceaccording to Embodiment 2.

FIGS. 6(a) to 6(c) are characteristic diagrams illustrating the impurityconcentration profile of an n-type FS layer in a semiconductor deviceaccording to Embodiment 3.

FIGS. 7(a) to 7(c) are characteristic diagrams illustrating anotherexample of the impurity concentration profile of the n-type FS layer inthe semiconductor device according to Embodiment 3.

FIGS. 8(a) to 8(c) are characteristic diagrams illustrating the relationbetween carrier concentration and the average range of protonirradiation in the related art.

FIG. 9 is a characteristic diagram illustrating a threshold voltage atwhich a voltage waveform starts to oscillate.

FIGS. 10(a) and 10(b) are diagrams illustrating the structure and netdoping concentration of the general IGBT.

FIG. 11 is a diagram illustrating an oscillation waveform when the IGBTis turned off.

FIG. 12 is a characteristic diagram illustrating the relation betweenthe range of protons and the acceleration energy of the protons in thesemiconductor device according to the invention.

FIG. 13 is a table illustrating the position conditions of a field stoplayer which a depletion layer reaches first in the semiconductor deviceaccording to the invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Hereinafter, a semiconductor device and a method for producing asemiconductor device according to exemplary embodiments of the inventionwill be described in detail with reference to the accompanying drawings.In the specification and the accompanying drawings, in the layers orregions having “n” or “p” appended thereto, an electron or a hole meansa majority carrier. In addition, symbols “+” and “−” added to n or pmean that impurity concentration is higher and lower than that of thelayer or the region without the symbols. In the description of thefollowing embodiments and the accompanying drawings, the same componentsare denoted by the same reference numerals and the description thereofwill not be repeated. The invention is not limited to the followingembodiments as long as it does not depart from the spirit and scopethereof.

Embodiment 1

FIG. 2 is a cross-sectional view schematically illustrating thestructure of a general IGBT. FIG. 2(a) is a schematic cross-sectionalview illustrating the IGBT including an n-type field stop (FS) layer 3which is formed by a general proton irradiation method according to therelated art. FIG. 2(b) shows the impurity concentration profile of then-type FS layer 3 which is measured by a known spreading resistance (SR)measurement method. However, in the schematic cross-sectional view ofthe IGBT, which is an example of the semiconductor device according tothe invention, the semiconductor device according to the invention hasthe same layer structure as that shown in FIG. 2(a). Therefore, thecross-sectional view shown in FIG. 2(a) is used in the description ofthe semiconductor device according to the invention. An IGBT produced bya semiconductor device production method according to the inventiondiffers from the IGBT produced by the method according to the relatedmethod in the impurity concentration profile of the n-type FS layer 3shown in FIG. 2(b). The impurity concentration profile of the n-type FSlayer 3 in the IGBT according to the invention will be described below.

FIG. 10 is a diagram illustrating net doping concentration and thestructure of the general IGBT. In the general IGBT shown in FIG. 10, aMOS gate structure including a p base layer 33, an n+emitter layer 2, pnjunction 23, an interlayer dielectric 41, a gate insulating film 43, anda gate electrode 42 is formed on one main surface of an n⁻ drift layer 1(high-resistance semiconductor layer) which is an n-type semiconductorsubstrate. In FIG. 2(a), for simplicity of illustration, the componentsof the MOS gate structure other than the n+ emitter layer 2 are notshown. The n-type FS layer 3 which has a higher impurity concentrationthan the n³¹ drift layer 1 and is formed by proton irradiation and a pcollector layer 4 which comes into contact with the surface (the othermain surface) of the n-type FS layer 3 are formed on the other mainsurface of the n³¹ drift layer 1. The n-type FS layer 3 has a pluralityof impurity concentration peaks (proton peaks), that is, a firstconcentration peak 6 a, a second concentration peak 6 b, and a thirdconcentration peak 6 c at different positions in the depth direction ofthe substrate. The distances of the proton peaks from an emitterelectrode 31 are, for example, 60 μm, 90 μm, and 115 μm. The emitterelectrode 31 which comes into contact with the p base layer 33 and then+ emitter layer 2 is formed on the front surface of the substrate. Acollector electrode 32 which comes into contact with the p collectorlayer 4 is formed on the rear surface of the substrate. In addition, ann-type leakage stop layer 38 is formed so as to come into contact withthe p collector layer 4. However, the n-type leakage stop layer 38 maynot be provided.

The method for forming the n-type FS layer 3 using proton irradiation isa known technique. However, as disclosed in Patent Literature 1 andPatent Literature 2, in order to form the donor layer (hereinafter,referred to as a hydrogen-related donor layer) using proton irradiation,the crystal defects generated by proton irradiation need to remain,without being recovered by an annealing process after the protonirradiation. In the above-mentioned method according to the related art,since the crystal defects remain in the n-type FS layer 3, the impurityconcentration peak (hereinafter, referred to as a proton peak) 6 a ofthe n-type FS layer 3 increases. However, since disorder 7 also remains,a problem, such as an increase in leakage current due to the disorder 7,is likely to arise.

The invention is characterized by an improvement in the protonirradiation method for suppressing the occurrence of the disorder 7 inthe n-type FS layer 3 when the n-type FS layer 3 is formed by protonirradiation. The other portions of the IGBT 10 (for example, a MOS gatestructure, an oxide film, a pn junction, an electrode, and a protectivefilm on the front surface of the substrate) can be formed by the sameproduction method as the known production method. Therefore, in thefollowing description, the detailed description of the known processesof the IGBT production method will not be repeated.

First, a semiconductor device production method according to Embodiment1 will be described with reference to FIG. 3, using an IGBT productionmethod as an example. FIG. 3 is a flowchart illustrating the outline ofthe semiconductor device production method according to Embodiment 1.First, a front surface forming process is performed which forms, forexample, a MOS gate structure including a p base layer (not shown), then+ emitter layer 2, a gate insulating film (not shown), and a gateelectrode (not shown) in a front surface of an n-type semiconductorsubstrate (wafer) using a general method (FIG. 3(a)).

Then, a front surface Al electrode process is performed which forms anemitter electrode (not shown) that is, for example, an aluminum (Al)electrode and commonly comes into conductive contact with the surfacesof both the p base layer and the n+ emitter layer 2 (FIG. 3(b)). Then, asurface protective film forming process is performed which forms apolyimide film, which is a surface protective film, on the front surfaceof the n-type semiconductor substrate (FIG. 3(c)). Then, a waferthinning process is performed which grinds the rear surface of then-type semiconductor substrate in order to reduce the thickness of then-type semiconductor substrate to a predetermined value that isdetermined by the relation with a breakdown voltage (FIG. 3(d)).

Then, a rear surface diffusion layer process is performed which implantsprotons and boron (B) ions into the ground rear surface of the n-typesemiconductor substrate a plurality of times and forms an n-type FSlayer 3 and a p collector layer 4 using an annealing process (FIG.3(e)). Then, a rear surface electrode process is performed which forms ametal film serving as a collector electrode that comes into conductivecontact with the surface of the p collector layer 4 using a vacuumsputtering method (FIG. 3(f)). In this way, an IGBT with an FS structureaccording to Embodiment 1 is completed.

The n-type FS layer 3 has an impurity concentration profile which has aplurality of impurity concentration peaks (proton peaks) at differentpositions in the depth direction of the substrate, due to the pluralityof proton irradiation operations in the rear surface diffusion layerprocess. Hereinafter, the proton peak formed by m-th proton irradiationis referred to as an m-th proton peak 6 n (m=1, 2, . . . , and n=a, b, .. . ). A method of performing proton irradiation a plurality of times inthe rear surface diffusion layer process will be described below.

Next, the impurity concentration profile of the n-type FS layer 3 willbe described. FIG. 1 is a characteristic diagram illustrating theimpurity concentration profile of the n-type FS layer in thesemiconductor device according to Embodiment 1. FIG. 1 shows theimpurity concentration profile of the n-type FS layer 3 after protonirradiation is performed from the rear surface of the n-typesemiconductor substrate and then an annealing process is performedduring the manufacture of the semiconductor device shown in FIG. 2(a).The vertical axis is the impurity concentration of the n-type FS layer 3and the horizontal axis is a depth from the rear surface of the n-typesemiconductor substrate. The impurity concentration profile of then-type FS layer 3 shown in FIG. 1 can be obtained by the known SR method(which holds for FIGS. 4 to 7). In general, the value of the mobility ofa silicon crystal is used as the value of mobility which is used toconvert resistivity and carrier concentration from spreading resistancein, for example, a measurement device. Therefore, the converted carrierconcentration is calculated to be lower than activation dopantconcentration, considering a reduction in the actual mobility.

FIG. 1 shows a change in the impurity concentration profile of then-type FS layer 3 when the proton irradiation and the annealing processare repeated until disorder is removed or the degree of disorder isreduced (which is holds for FIGS. 5 to 7). FIG. 1(a) shows the impurityconcentration profile of the n-type FS layer 3 after first protonirradiation and an annealing process. As shown in FIG. 1(a), after thefirst proton irradiation and the annealing process, one mountain (firstproton peak 6 a) with high impurity concentration is formed at a deepposition from the rear surface of the substrate in the n-type FS layer 3and a region (a portion surrounded by a dotted line) of the disorder 7in which impurity concentration is significantly lower than the impurityconcentration of the silicon substrate (semiconductor substrate) isdisposed on the irradiation surface (the rear surface side of thesubstrate) side. That is, in FIG. 1(a), when the disorder 7 occurs, areduction in mobility is reflected in the impurity concentrationconverted from spreading resistance and the impurity concentration isreduced.

FIG. 1(b) shows the impurity concentration profile of the n-type FSlayer 3 after second and third proton irradiation operations and theannealing processes. FIG. 1(b) shows the impurity concentration profileof the n-type FS layer 3 when the second proton peak 6 b is formed at aposition which is closer to the rear surface of the substrate than theintermediate position between the rear surface of the substrate and theposition of the first proton peak 6 a. In FIG. 1(b), the third protonpeak 6 c is also formed at a position close to the rear surface of thesubstrate. As shown in FIG. 1(b), after the second and third protonirradiation operations and the annealing processes, the disorder 7 inwhich impurity concentration is significantly lower than the impurityconcentration of the silicon substrate remains in a region between thefirst proton peak 6 a and the second proton peak 6 b. As shown in FIGS.1(a) and 1(b), in the case in which the region of the disorder 7 isformed in the n-type FS layer 3, when a depletion layer which is spreadfrom a breakdown voltage main junction of the IGBT 10 at the time theIGBT 10 is turned on goes into the region of the disorder 7, thedisorder 7 causes the generation of a leakage current and the amount ofleakage current increases, which is not preferable.

Next, a proton irradiation method according to Embodiment 1 capable ofsuppressing a reduction (a reduction in mobility) in the impurityconcentration of the n-type FS layer 3 shown in FIGS. 1(a) and 1(b) willbe described in detail. FIG. 1(c) shows the impurity concentrationprofile of the n-type FS layer 3 when an irradiation position isadjusted, the second proton irradiation and the annealing process areperformed, and then the third proton irradiation and the annealingprocess are performed. In Embodiment 1, after the first protonirradiation for forming the n-type FS layer 3 on the rear surface of theIGBT 10 is performed, irradiation conditions, such as protonacceleration energy, are changed, proton irradiation is sequentiallyperformed a plurality of times at the position that is closer to therear surface of the substrate than the position of the first proton peak6 a from the rear surface of the substrate, and the annealing process isperformed. That is, after the first proton irradiation shown in FIG.1(a), the hydrogen-related donor layer is formed in the n-type FS layer3 by a plurality of proton irradiation operations to compensate for thehydrogen-related donor concentration of the n-type FS layer 3. In thisway, as shown in FIG. 1(c), it is possible to reduce the degree of thedisorder 7 (the degree of reduction in impurity concentration due to thedisorder 7) formed by the first proton irradiation or remove thedisorder 7. It is presumed that the degree of the disorder 7 is reducedbecause a dangling bond which is present in a portion with the largestdisorder 7 is terminated by the implanted proton (that is, a hydrogenatom).

Specifically, first, after the first proton irradiation, thedistribution of the disorder 7 formed by the first proton irradiation ismeasured by the SR method. Then, the second and third proton irradiationoperations are performed at the position that is closer to the rearsurface of the substrate than the position of the first proton peak 6 ashown in FIG. 1(a), that is, the second and third irradiation positionsrepresented by arrows in FIG. 1(a) in the impurity concentration profileof the n-type FS layer 3 shown in FIG. 1(a), in order to reduce thedegree of the disorder 7 or to remove the disorder 7 on the basis of thedepth of the disorder 7 from the rear surface of the substrate. Thefirst to third proton irradiation operations are performed at differentacceleration energy levels. As described above, when the second protonpeak 6 b is formed at the position that is closer to the rear surface ofthe substrate than the intermediate position between the rear surface ofthe substrate and the first proton peak 6 a, the disorder 7 remains atthe intermediate position between the first proton peak 6 a and thesecond proton peak 6 b (FIG. 1(b)). This is because the position of thesecond proton peak 6 b is not appropriate. Specifically, a distance bbetween the position of the first proton peak 6 a and the second protonpeak 6 b is more than a distance a between the position of the secondproton peak 6 b and the irradiation surface (the rear surface of thesubstrate). As a result, the doping concentration effect by the secondproton peak 6 b (hydrogen-related donor layer) is reduced.

As can be seen from FIG. 1(c), when the second proton irradiationposition is the position of the disorder 7 generated by the first protonirradiation or is in the vicinity of the position of the disorder 7, itis possible to remove almost all of the disorder 7. Therefore, as shownin FIG. 1(c), the second proton irradiation position is adjusted toimprove the doping compensation effect by the second proton peak 6 b(hydrogen-related donor layer). For example, the distance between thesecond proton irradiation position and the rear surface of the substratemay be equal to or more than half of the distance between the firstproton irradiation position and the rear surface of the substrate. Thatis, the average range of the second proton irradiation may be equal toor more than half of the average range of the first proton irradiation.The average range means the depth of the peak concentration position ofthe impurity concentration distribution of the n-type FS layer 3, whichis represented by a Gaussian distribution, from the rear surface of thesubstrate. Specifically, the average range is a depth from the rearsurface of the substrate to the proton peak position. A method ofsetting the second proton irradiation position will be described below.The difference between the second proton irradiation position shown inFIG. 1(c) and the second proton irradiation position shown in FIG. 1(b)is represented by a white arrow in FIG. 1(c). An example of the detailedion implantation conditions of three proton irradiation operations whenthe impurity concentration profile of the n-type FS layer 3 without thedisorder 7 is obtained will be described below. However, the ionimplantation conditions are not particularly limited.

The acceleration energy and dose of proton irradiation (that is, thefirst proton irradiation) for forming the first proton peak 6 a are 2.3MeV and 3×10¹³ cm², respectively. The acceleration energy and dose ofproton irradiation (that is, the second proton irradiation) for formingthe second proton peak 6 b are 1.5 MeV and 3×10¹³ cm², respectively. Theacceleration energy and dose of proton irradiation (that is, the thirdproton irradiation) for forming the third proton peak 6 c are 0.5 MeVand 2×10¹⁴ cm², respectively. The average range of the third protonirradiation is, for example, from 6 μm to 15 μm from the rear surface ofthe substrate. It is preferable that, after the proton irradiation, theannealing process be performed, for example, at a temperature of about450° C. for 5 hours in a reduction atmosphere (for example, a hydrogenatmosphere in which hydrogen concentration is 3% or a nitrogenatmosphere including hydrogen).

When irradiation is performed in four stages, instead of three stages,the detailed ion implantation conditions of the fourth protonirradiation are as follows. The acceleration energy and dose of thefirst proton irradiation are 1.5 MeV and 2×10¹³/cm², respectively. Theacceleration energy and dose of the second proton irradiation are 1.1MeV and 2×10¹³/cm², respectively. The acceleration energy and dose ofthe third proton irradiation are 0.8 MeV and 5×10¹³/cm², respectively.The acceleration energy and dose of proton irradiation (that is, thefourth proton irradiation) for forming a fourth proton peak are 0.4 MeVand 1×10¹⁴/cm², respectively. For example, it is preferable that, afterthe proton irradiation, the annealing process be performed in areduction atmosphere at a temperature of about 380° C. to 450° C. four 5hours.

As shown in FIG. 1(c), the positional relationship between the firstproton peak 6 a and the second proton peak 6 b is an important point ofthe invention. As shown in FIG. 1(a), only the formation of the firstproton peak 6 a causes the disorder 7 to be formed closer to theirradiation surface (the rear surface of the substrate) than theposition of the first proton peak 6 a. At the position where the degreeof the disorder 7 is the largest, a reduction in mobility is the largestand the impurity concentration measured by the SR method is the lowest.That is, the degree of the disorder 7 is the largest at the positionthat is closer to the first proton peak 6 a than the intermediateposition between the rear surface of the substrate and the first protonpeak 6 a. The reason is as follows. When the hydrogen ion (proton)implanted into the silicon substrate collides with a silicon atom, givesenergy to the silicon atom, and is decelerated while forming distortion,that is, the disorder 7 in the silicon lattice, the position of therange Rp of the proton and the silicon lattice which is arranged in thevicinity of the position are a region which receives the largest amountof energy from the protons.

In particular, the region in which the silicon lattice receives thelargest amount of energy from the protons radiated to the siliconsubstrate is disposed at the position where mobility is the lowest, thatis, the position where carrier concentration is the lowest in the regionin which the disorder 7 occurs. Therefore, the position where the secondproton peak 6 b is formed by the second proton irradiation may be theposition where the degree of the disorder 7 is the largest by the firstproton irradiation or in the vicinity of the position. Specifically, theposition where the second proton peak 6 b is formed by the second protonirradiation is closer to the position of the first proton peak 6 a fromthe rear surface of the substrate than the intermediate position betweenthe rear surface of the substrate and the position of the first protonpeak 6 a. The determination of the position of the second proton peak 6b enables the disorder 7 between the range Rp of the second protonirradiation and the range Rp of the first proton irradiation to supplydefects required to generate donors when the hydrogen-related donor isformed in the vicinity of the range Rp of the second proton irradiation.As a result, the defects in the region of the disorder 7 compensate forthe formation of donors to accelerate the formation of donors and thedisorder 7 is removed.

Therefore, it is preferable that the difference between the range Rp ofthe first proton irradiation and the range Rp of the second protonirradiation be less than the range Rp of the second proton irradiation.It is more preferable that the difference between the range Rp of thefirst proton irradiation and the range Rp of the second protonirradiation be equal to or less than half of the range Rp of the secondproton irradiation, in order to reliably remove the disorder 7.Alternatively, the distance b between the position of the first protonpeak 6 a (the peak position of carrier concentration measured by the SRmethod) and the position of the second proton peak 6 b is preferablyless than the distance a between the position of the second proton peak6 b and the rear surface of the substrate, and more preferably equal toor less than half of the distance a.

Alternatively, the difference between the distance from the position(the position where mobility is the lowest) where the carrierconcentration measured by the SR method is the lowest in the disorder 7generated by the formation of the first proton peak 6 a to the rearsurface of the substrate and the range Rp of the second protonirradiation is preferably less than the range Rp of the second protonirradiation and more preferably equal to or less than half of the rangeRp of the second proton irradiation. In addition, the distance b betweenthe position where the carrier concentration measured by the SR methodis the lowest and the position of the second proton peak 6 b ispreferably less than the distance a between the position of the secondproton peak 6 b and the rear surface of the substrate, and morepreferably equal to or less than half of the distance a.

It is preferable that the total number of proton peaks formed in then-type FS layer 3 be equal to or more than 3. The reason is as follows.Among a plurality of proton peaks, a proton peak at the shallowestposition (that is, the proton peak closest to the rear surface of thesubstrate) is formed at a depth less than 5 μm from the rear surface ofthe substrate such that the depletion layer reaches the p collectorlayer 4 (a desired field stop function is obtained). Therefore, when thetotal number of proton peaks formed in the n-type FS layer 3 is two, thesecond proton peak formed at the shallowest position is 5 μm away fromthe rear surface of the substrate in order to obtain the desired fieldstop function and the first proton peak formed at the deepest positionis, for example, about 50 μm away from the rear surface of thesubstrate. In this case, since the distance between the first protonpeak and the second proton peak is 45 μm, disorder is likely to occur.Therefore, it is preferable to form one proton peak between the protonpeak which is formed at a shallow position from the rear surface of thesubstrate and the proton peak which is formed at a deep position fromthe rear surface of the substrate. In this way, as described above, itis possible to compensate for the hydrogen-related donor concentrationof the n-type FS layer 3 and suppress a reduction in mobility. Inaddition, it is possible to remove disorder.

FIG. 4 is a characteristic diagram illustrating another example of theimpurity concentration profile of the n-type FS layer in thesemiconductor device according to Embodiment 1. As described above, thefourth proton irradiation may be performed for the disorder 7 whichremains between the position of the first proton peak 6 a and theposition of the second proton peak 6 b, without adjusting the secondproton irradiation position. Specifically, as shown in FIGS. 1(a) and1(b), the first to third proton peaks 6 a to 6 c are formed by the firstto third proton irradiation operations. When the second protonirradiation position is not adjusted, the disorder 7 remains between theposition of the first proton peak 6 a and the position of the secondproton peak 6 b, as shown in FIG. 1(b).

The fourth proton irradiation is further performed for the disorder 7which remains between the position of the first proton peak 6 a and theposition of the second proton peak 6 b. In this way, as shown in FIG. 4,the fourth proton peak 6 d is formed between the position of the firstproton peak 6 a and the position of the second proton peak 6 b and it ispossible to remove the entire disorder 7 in the n-type FS layer 3 or toreduce the degree of the disorder 7. An example of the detailed ionimplantation conditions of the fourth proton irradiation for obtainingthe impurity concentration profile of the n-type FS layer 3 without thedisorder 7 will be described below. However, the ion implantationconditions are not particularly limited.

The acceleration energy and dose of the first proton irradiation are 1.5MeV and 1×10¹³/cm², respectively. The acceleration energy and dose ofthe second proton irradiation are 1.1 MeV and 1×10¹³/cm², respectively.The acceleration energy and dose of the third proton irradiation are 0.8MeV and 2×10¹³/cm², respectively. The acceleration energy and dose ofproton irradiation (that is, the fourth proton irradiation) for formingthe fourth proton peak 6 d are 0.4 MeV and 3×10¹⁴/cm², respectively. Theaverage range of the fourth proton irradiation is, for example, fromabout 6 μm to 15 μm from the rear surface of the substrate. It ispreferable that, after the proton irradiation, an annealing process beperformed, for example, at a temperature of about 380° C. for 5 hours ina reduction atmosphere (for example, a hydrogen atmosphere in whichhydrogen concentration is 3% or a nitrogen atmosphere includinghydrogen).

As described above, according to Embodiment 1, the range of the protonirradiation is set to the above-mentioned conditions and the protonirradiation is performed a plurality of times, or the proton irradiationis performed a plurality of times such that the distance between theproton peaks formed by each proton irradiation operation satisfies theabove-mentioned conditions. Therefore, it is possible to remove thedisorder which is generated in the silicon substrate by the protonirradiation method according to the related art or to reduce the degreeof disorder such that the disorder does not have an adverse effect onthe element characteristics. As a result, it is possible to form ann-type FS layer in which no disorder occurs or the degree of disorder isreduced. Furthermore, it is possible to form an n-type FS layer with adesired field stop function in which impurity concentration (carrierconcentration) is not greatly reduced or a reduction in impurityconcentration is small. Therefore, it is possible to produce asemiconductor device with an FS structure capable of preventingdeterioration of characteristics, such as a leakage current.

Embodiment 2

FIG. 5 is a characteristic diagram illustrating the impurityconcentration profile of an n-type FS layer in a semiconductor deviceaccording to Embodiment 2. A semiconductor device production methodaccording to Embodiment 2 differs from the semiconductor deviceproduction method according to Embodiment 1 in that a plurality ofproton irradiation operations for removing disorder 17 or reducing thedegree of the disorder 17 are sequentially performed from a deepposition to a shallow position from the rear surface of a substrate inthe region of the disorder 17.

The semiconductor device produced by the semiconductor device productionmethod according to Embodiment 2 is, for example, an IGBT shown in FIG.2(a), similarly to Embodiment 1. The semiconductor device productionmethod according to Embodiment 2 is the same as the semiconductor deviceproduction method according to Embodiment 1 except for a protonirradiation method for forming an n-type FS layer 3. Therefore, only theproton irradiation method for forming the n-type FS layer 3 will bedescribed (which holds for Embodiment 3).

The proton irradiation method according to Embodiment 2 will bedescribed in detail. FIGS. 5(a) to 5(c) show the impurity concentrationprofile of the n-type FS layer 3 after the first to third protonirradiation operations and annealing processes. As shown in FIG. 5(a), afirst proton peak 16 a is formed at a predetermined depth from the rearsurface of the substrate by the first proton irradiation to form theregion (a portion surrounded by a dotted line) of the disorder 17between the rear surface of the substrate and the position of the firstproton peak 16 a, similarly to Embodiment 1.

Then, the distribution of the disorder 17 formed by the first protonirradiation is measured by the SR method. Then, the second protonirradiation is performed at a deep position (for example, a secondirradiation position represented by an arrow in FIG. 5(a)) from the rearsurface of the substrate in the region of the disorder 17 formed by thefirst proton irradiation. Then, as shown in FIG. 5(b), a second protonpeak 16 b is formed at a deep position from the rear surface of thesubstrate in the region of the disorder 17, which makes it possible toremove the disorder 17 at a deep position from the rear surface of thesubstrate or to reduce the degree of the disorder 17.

When the region of the disorder 17 remains between the rear surface ofthe substrate and the position of the second proton peak 16 b, the thirdproton irradiation is performed for the region (for example, a thirdirradiation position represented by an arrow in FIG. 5(b)) of thedisorder 17 which remains between the rear surface of the substrate andthe position of the second proton peak 16 b. Then, as shown in FIG.5(c), a third proton peak 16 c is formed at a shallow position from therear surface of the substrate, which makes it possible to remove theentire disorder 17 of the n-type FS layer 3 or to reduce the degree ofthe disorder 17.

In the proton irradiation method according to Embodiment 2, the thirdproton irradiation is performed to remove the entire disorder 17 of then-type FS layer 3 or to reduce the degree of the disorder 17. However,when the region of the disorder 17 remains between the rear surface ofthe substrate and the position of the third proton peak 16 c, the fourthproton irradiation may be performed at a deep position in the region ofthe disorder 17 which remains between the rear surface of the substrateand the position of the third proton peak 16 c.

As such, when the region of the disorder 17 remains between the rearsurface of the substrate and the position of an m-th proton peak 16 n,an (m+1)-th proton irradiation operation is performed at a deep positionfrom the rear surface of the substrate in the remaining region of thedisorder 17 and this process is repeated (m=1, 2, . . . , and n=a, b, .. . ). Then, the region of the disorder 17 which remains at a shallowposition from the rear surface of the substrate is gradually reduced. Anexample of the detailed ion implantation conditions of three protonirradiation operations when the impurity concentration profile of then-type FS layer 3 without the disorder 17 is obtained will be describedbelow. However, the ion implantation conditions are not particularlylimited.

The acceleration energy and dose of the first proton irradiation are 2.0MeV (average range: 47.7 μm) and 3×10¹³/cm², respectively. Theacceleration energy and dose of the second proton irradiation is 1.5 MeV(average range: 30.3 μm) and 3×10¹³/cm², respectively. The accelerationenergy and dose of the third proton irradiation are 0.5 MeV (averagerange: 6.0 μm) and 2×10¹⁴/cm², respectively. It is preferable that,after the proton irradiation, an annealing process be performed, forexample, at a temperature of about 380° C. for 5 hours in a reductionatmosphere (for example, a hydrogen atmosphere in which hydrogenconcentration is 3% or a nitrogen atmosphere including hydrogen). Theannealing process after the proton irradiation may be performed underthe conditions of a temperature of about 300° C. to 450° C. and aprocessing time of about 1 to 10 hours according to the requiredspecifications of the n-type FS layer 3.

As described above, according to Embodiment 2, it is possible to obtainthe same effect as that in Embodiment 1.

Embodiment 3

FIG. 6 is a characteristic diagram illustrating the impurityconcentration profile of an n-type FS layer in a semiconductor deviceaccording to Embodiment 3. A semiconductor device production methodaccording to Embodiment 3 differs from the semiconductor deviceproduction method according to Embodiment 1 in that a plurality ofproton irradiation operations for removing disorder 27 or reducing thedegree of the disorder 27 are sequentially performed from a shallowposition to a deep position from the rear surface of a substrate in theregion of the disorder 27.

A proton irradiation method according to Embodiment 3 will be described.FIGS. 6(a) to 6(c) show the impurity concentration profile of an n-typeFS layer 3 after the first to third proton irradiation operations andannealing processes. As shown in FIG. 6(a), a first proton peak 26 a isformed at a predetermined depth from the rear surface of the substrateby the first proton irradiation to form the region (a portion surroundedby a dotted line) of the disorder 27 between the rear surface of thesubstrate and the position of the first proton peak 26 a, similarly toEmbodiment 1.

Then, the second proton irradiation is performed at a shallow position(for example, a second irradiation position represented by an arrow inFIG. 6(a)) from the rear surface of the substrate in the region of thedisorder 27 formed by the first proton irradiation. Then, as shown inFIG. 6(b), a second proton peak 26 b is formed at a shallow positionfrom the rear surface of the substrate in the region of the disorder 27.Therefore, it is possible to remove the disorder 27 at this position orto reduce the degree of the disorder 27.

When the region of the disorder 27 remains between the position of thefirst proton peak 26 a and the position of the second proton peak 26 b,the third proton irradiation is performed for the region (for example, athird irradiation position represented by an arrow in FIG. 6(b)) of thedisorder 27 which remains between the position of the first proton peak26 a and the position of the second proton peak 26 b. Then, as shown inFIG. 6(c), a third proton peak 26 c is formed between the position ofthe first proton peak 26 a and the position of the second proton peak 26b. Therefore, it is possible to remove the entire disorder 27 of then-type FS layer 3 or to reduce the degree of the disorder 27.

FIG. 7 is a characteristic diagram illustrating another example of theimpurity concentration profile of the n-type FS layer in thesemiconductor device according to Embodiment 3. As shown in FIGS. 7(a)and 7(b), when the region of the disorder 27 remains between theposition of the first proton peak 26 a and the position of the thirdproton peak 26 c after the second and third proton peaks 26 b and 26 care formed by the second and third proton irradiation operations, thefourth proton irradiation may be performed for the region of theremaining disorder 27 (for example, a fourth irradiation positionrepresented by an arrow in FIG. 7(b)).

As shown in FIG. 7(c), a fourth proton peak 26 d is formed between theposition of the first proton peak 26 a and the position of the thirdproton peak 26 c by the fourth proton irradiation. Therefore, it ispossible to remove the entire disorder 27 of the n-type FS layer 3 or toreduce the degree of the disorder 27.

As such, when the region of the disorder 27 remains between the positionof the first proton peak 26 a and the position of an (m+1)-th protonpeak 26 n after an (m+1)-th proton irradiation operation, an (m+2)-thproton irradiation operation is performed for the region of the disorder27 between the position of the first proton peak 26 a and the positionof the (m+1)-th proton peak 26 n (m=2, 3, . . . , and n=b, c, . . . ).Then, the region of the disorder 27 which remains between the positionsof the proton peaks is gradually reduced.

As described above, according to Embodiment 3, it is possible to obtainthe same effect as that in Embodiment 1.

Embodiment 4

The preferred position of a first proton peak will be described below.FIG. 11 shows an oscillation waveform when an IGBT is turned off. When acollector current is equal to or less than one-tenth of the ratedcurrent, in some cases, the number of carriers accumulated is small andoscillation occurs before the IGBT is turned off. The collector currentis fixed to a certain value and the IGBT is turned off by a differentpower supply voltage VCC. In this case, when the power supply voltageVCC is greater than a predetermined value, it is greater than the peakvalue of a general overshoot voltage in the voltage waveform between thecollector and the emitter and an additional overshoot occurs. Theadditional overshoot (voltage) serves as a trigger and the subsequentwaveform oscillates. When the power supply voltage VCC is greater thanthe predetermined value, the additional overshoot voltage furtherincreases and the subsequent oscillation amplitude also increases. Assuch, a threshold voltage at which the voltage waveform starts tooscillate is referred to as an oscillation start threshold value VRRO.When the oscillation start threshold value VRRO increases, the IGBT doesnot oscillate when it is turned off, which is preferable.

The oscillation start threshold value VRRO depends on the position ofthe first proton peak that a depletion layer (strictly, a space chargeregion since there is a hole) which is spread from the pn junctionbetween a p-type base layer and an n⁻ drift layer of the IGBT to the n⁻drift layer reaches first, among a plurality of proton peaks. The reasonis as follows. When the depletion layer is spread from the p-type baselayer in the front surface to the n⁻ drift layer at the time the IGBT isturned off, the end of the depletion layer reaches the first n-type FSlayer and the spreading of the depletion layer is suppressed. The sweepof the accumulated carriers is weakened. As a result, the depletion ofthe carriers is suppressed and oscillation is suppressed.

When the IGBT is turned off, the depletion layer is spread from the pnjunction between the p base layer and the n⁻ drift layer toward thecollector electrode in the depth direction. Therefore, the peak positionof the n-type FS layer which the end of the depletion layer reachesfirst is the n-type FS layer which is closest to the pn junction. Here,the thickness of the n⁻ semiconductor substrate (the thickness of aportion interposed between the emitter electrode and the collectorelectrode) is W0 and the depth of the peak position of the n-type FSlayer which the end of the depletion layer reaches first from theinterface between the collector electrode and the rear surface of the n⁻semiconductor substrate (hereinafter, referred to as a distance from therear surface) is X. A distance index L is introduced. The distance indexL is represented by the following Expression (1).

$\begin{matrix}{L = \sqrt{\frac{ɛ_{S}V_{rate}}{q\left( {\frac{J_{F}}{{qv}_{sat}} + N_{d}} \right)}}} & {{Expression}\mspace{14mu}(1)}\end{matrix}$

The distance index L represented by the above-mentioned Expression (1)is an index indicating the distance of the end (depletion layer end) ofthe depletion layer (space charge region), which is spread from the pnjunction to n⁻ drift layer 21, from the pn junction when a voltage VCEbetween the collector and the emitter is a power supply voltage VCC, atthe time the IGBT is turned off. In a fraction in the square root, adenominator indicates the space charge density of the space chargeregion (simply, the depletion layer) when the IGBT is turned off. Theknown Poisson equation is represented by divE=ρ/∈, in which E iselectric field intensity, ρ is space charge density, and ρ=q(p−n+Nd−Na)is established. Here, q is an elementary charge, p is holeconcentration, n is electron concentration, Nd is donor concentration,Na is acceptor concentration, and ∈_(s) is the permittivity of asemiconductor.

The space charge density ρ is described by the hole concentration ppassing through the space charge region (depletion layer) when the IGBTis turned off and the average donor concentration Ndm of the n⁻ driftlayer. The electron concentration is lower than these concentrations soas to be negligible and there is no acceptor. Therefore, ρ≈q(p+Ndm) isestablished. In this case, the hole concentration p is determined by abreaking current of the IGBT. In particular, since it is assumed thatthe element is being energized, the rated current density is expressedby p=J_(F)/(qv_(sat)). J_(F) is the rated current density of the elementand v_(sat) is a saturated velocity in which the speed of the carrier issaturated with predetermined electric field intensity.

The Poisson equation is integrated with respect to the distance x twotimes and the voltage V satisfies E=−gradV (the relationship between theknown electric field E and the voltage V). Therefore, under appropriateboundary conditions, V=(½)(ρ/∈)x² is established. The length x of thespace charge region obtained when the voltage V is half of the ratedvoltage BV is the distance index L. This is because an operation voltage(power supply voltage), which is the voltage V, is about half of therated voltage in the actual device such as an inverter. When the dopingconcentration of the FS layer is higher than that of the n⁻ drift layer,the n-type FS layer prevents the expansion of the space charge regionwhich is spread at the time the IBGT is turned off. In a case in whichthe collector current of the IGBT starts to be reduced from the breakingcurrent due to the turn-off of a MOS gate, when the peak position of theFS layer which the depletion layer reaches first is in the space chargeregion, it is possible to suppress the expansion of the space chargeregion, with the accumulated carriers remaining in the n⁻ drift layer.Therefore, the sweep of the remaining carriers is suppressed.

In the actual turning-off operation, for example, when a motor is drivenby a known PWM inverter with an IGBT module, the power supply voltage orthe breaking current is not fixed, but is variable. Therefore, in thiscase, the preferred peak position of the n-type FS layer which thedepletion layer reaches first needs to have a given width. According tothe results of the inventors' research, the distance X of the peakposition of the n-type FS layer which the depletion layer reaches firstfrom the rear surface is as shown in FIG. 13. FIG. 13 is a tableillustrating the position conditions of the FS layer which the depletionlayer reaches first in the semiconductor device according to theinvention. FIG. 13 shows the distance X of the peak position of the FSlayer which the end of the depletion layer reaches first from the rearsurface at a rated voltage of 600 V to 6500 V. Here, X=W0−γL isestablished and γ is a coefficient. FIG. 19 shows the distance X when γis changed from 0.7 to 1.6.

As shown in FIG. 13, at each rated voltage, the element (IGBT) is safelydesigned so as to have a breakdown voltage which is about 10% higherthan the rated voltage. As shown in FIG. 13, the total thickness of then⁻ semiconductor substrate (the thickness during a finishing processafter the n⁻ semiconductor substrate is thinned by, for example,grinding) and the average resistivity of the n⁻ drift layer are set suchthat an on-voltage or turn-off loss is sufficiently reduced. The term‘average’ means the average concentration and resistivity of the entiren⁻ drift layer including the FS layer. The rated current density has thetypical value shown in FIG. 13 due to the rated voltage. The ratedcurrent density is set such that energy density which is determined bythe product of the rated voltage and the rated current density has asubstantially constant value and has nearly the value shown in FIG. 13.When the distance index L is calculated using these values according theabove-mentioned Expression (1), the value shown in FIG. 13 is obtained.The distance X of the peak position of the n-type FS layer which the endof the depletion layer reaches first from the rear surface is obtainedby subtracting the product of the distance index L and γ, which is inthe range of 0.7 to 1.6, from the thickness W0 of the n⁻ semiconductorsubstrate.

The distance X of the peak position of the FS layer which the end of thedepletion layer reaches first from the rear surface, at which turn-offoscillation is sufficiently suppressed, is as follows with respect tothe distance index L and the thickness W0 of the n⁻ semiconductorsubstrate. FIG. 9 is a characteristic diagram illustrating a thresholdvoltage at which the voltage waveform starts to oscillate. FIG. 9 showsthe dependence of V_(RRO) on γ at some typical rated voltages V_(rate)(600 V, 1200 V, and 3300 V). In FIG. 9, the vertical axis indicates avalue obtained by standardizing VRRO with the rated voltage V_(rate). Ascan be seen from FIG. 9, V_(RRO) can be rapidly increased at three ratedvoltages when γ is equal to or less than 1.4. When γ is in the range of0.8 to 1.3, it is possible to sufficiently increase VRRO at any ratedvoltage. More preferably, when γ is in the range of 0.9 to 1.2, it ispossible to maximize V_(RRO).

The important point in FIG. 9 is that the range of γ capable ofsufficiently increasing V_(RRO) is substantially the same (0.8 to 1.3)at all rated voltages. The reason is that it is most effective to setthe range of the distance X of the peak position of the n-type FS layerwhich the depletion layer reaches first from the rear surface to becentered on W0−L (that is, γ=1). That is, this characteristic is causedby the nearly constant value of the product of the rated voltage and therated current density. Therefore, when the distance X of the peakposition of the n-type FS layer which the end of the depletion layerreaches first from the rear surface is set in the above-mentioned range,the accumulated carriers can sufficiently remain in the IGBT at the timethe IGBT is turned off and it is possible to suppress the oscillationphenomenon at the time the IGBT is turned off. Therefore, for thedistance X of the peak position of the n-type FS layer which the end ofthe depletion layer reaches first from the rear surface, it ispreferable that the coefficient γ of the distance index L be in theabove-mentioned range at any rated voltage. In this way, it is possibleto effectively suppress the oscillation phenomenon at the time the IGBTis turned off.

The acceleration energy of protons may be determined from the followingcharacteristic graph shown in FIG. 12 in order to form the n-type FSlayer with the peak position which the depletion layer first reaches andwhich is at the distance X from the rear surface, using protonirradiation, such that the above-mentioned range of γ is satisfied inpractice.

The results of the inventors' research proved that, for the range Rp(the peak position of the n-type FS layer) of protons and theacceleration energy E of protons, when the logarithm log(Rp) of therange Rp of the protons was x and the logarithm log(E) of theacceleration energy E of the protons was y, x and y satisfied thefollowing relationship represented by Expression (2).y=−0.0047x ⁴+0.0528x ³−0.2211x ²+0.9923x+5.0474   Expression (2)

FIG. 12 is a characteristic graph indicating the above-mentionedExpression (2). FIG. 12 is a characteristic diagram illustrating therelationship between the range of the protons and the accelerationenergy of the protons in the semiconductor device according to theinvention. FIG. 12 shows the acceleration energy of the protons forobtaining the desired range of the protons. In FIG. 12, the horizontalaxis is the logarithm log(Rp) of the range Rp of the protons andindicates the range Rp (μm) corresponding to the parentheses below theaxis value of log(Rp). In addition, the vertical axis is the logarithmlog(E) of the acceleration energy E of the protons and indicates theacceleration energy E of the protons corresponding to the parentheses onthe left side of the axis value of log(E). The above-mentionedExpression (2) is obtained by fitting the value of the logarithm log(Rp)of the range Rp of the protons and the value of the logarithm log(E) ofthe acceleration energy with a four-order polynomial of x (=log(Rp)).

The following relationship may be considered between the actualacceleration energy E′ and an average range Rp′ (proton peak position)which is actually obtained by the spreading resistance (SR) method whenthe above-mentioned fitting equation is used to calculate theacceleration energy E of proton irradiation from the desired averagerange Rp of protons and to set the acceleration energy E and protons areimplanted into silicon. When the actual acceleration energy E′ is in therange of about □5% of the calculated value E of the acceleration energy,the actual average range Rp′ is within the range of about +/−5% of thedesired average range Rp and is in a measurement error range. Therefore,the influence of a variation in the actual average range Rp′ from thedesired average range Rp on the electrical characteristics of the IGBTis so small to be negligible. When the actual acceleration energy E′ isin the range of +/−5% of the calculated value E, it is possible todetermine that the actual average range Rp′ is substantially equal tothe set average range Rp. In the actual accelerator, since theacceleration energy E and the average range Rp are both in theabove-mentioned ranges (+/−5%), it is considered that the actualacceleration energy E′ and the actual average range Rp′ follow theabove-mentioned fitting equation represented by the desired averagerange Rp and the calculated value E and no problem occurs.

The above-mentioned Expression (2) is used to calculate the accelerationenergy E of the protons required to obtain the desired range Rp of theprotons. The acceleration energy E of each proton for forming the FSlayer is also calculated by the above-mentioned Expression (2) and iswell matched with the value which is measured from a sample by the knownspreading resistance measurement method (SR method) after protonirradiation is performed with the above-mentioned acceleration energy.Therefore, when the above-mentioned Expression (2) is used, it ispossible to predict the required acceleration energy E of protons withhigh accuracy, on the basis of the range Rp of the protons.

As described above, according to Embodiment 4, it is possible to obtainthe same effect as that in Embodiment 1.

In the above-mentioned embodiment, the IGBT is given as an example, butthe invention is not limited thereto. For example, the invention can beapplied to a diode. In addition, the invention can be applied tosemiconductor devices with a breakdown voltage of, for example, 600 V,1200 V, 1700 V, 3300 V, 4500 V, and 6000 V.

As described above, the semiconductor device and the method forproducing the semiconductor device according to the invention are usefulas power semiconductor devices used for power conversion apparatusessuch as converters or inverters.

Thus, a semiconductor device has been described according to the presentinvention. Many modifications and variations may be made to thetechniques and structures described and illustrated herein withoutdeparting from the spirit and scope of the invention. Accordingly, itshould be understood that the methods and devices described herein areillustrative only and are not limiting upon the scope of the invention.

REFERENCE SIGNS LIST

1 n⁻ drift layer (high-resistance semiconductor layer)

2 n+ emitter layer

3 n-type fs layer

4 p collector layer

6 a, 16 a, 26 a first proton peak

6 b, 16 b, 26 b second proton peak

6 c, 16 c, 26 c third proton peak

6 d, 26 d fourth proton peak

7 disorder

10 IGBT

What is claimed is:
 1. A method for producing a semiconductor deviceincluding a breakdown voltage holding pn junction that is provided inone main surface of an n-type semiconductor substrate and an n-typefield stop layer that is provided in the other main surface of then-type semiconductor substrate, has a lower resistance than the n-typesemiconductor substrate, and suppresses the spreading of a depletionlayer from the breakdown voltage holding pn junction, comprising: aproton irradiation step of repeatedly performing proton irradiation aplurality of times from the other main surface of the n-typesemiconductor substrate to form the n-type field stop layer in the othermain surface of the n-type semiconductor substrate, wherein, in theproton irradiation step, whenever the proton irradiation is repeated,the next proton irradiation is performed so as to compensate for areduction in mobility due to a disorder which remains after the previousproton irradiation.
 2. The method for producing a semiconductor deviceaccording to claim 1, wherein, in the proton irradiation step, anirradiation depth in the next proton irradiation is less than anirradiation depth in the previous proton irradiation.
 3. The method forproducing a semiconductor device according to claim 2, wherein, in theproton irradiation step, the second or subsequent proton irradiation isrepeatedly performed on the basis of the position of the disorder formedby the previous proton irradiation.
 4. The method for producing asemiconductor device according to claim 3, wherein, in the protonirradiation step, the acceleration energy and dose of the protonirradiation are adjusted such that a portion in which an impurityconcentration in the n-type field stop layer is reduced by the disorderformed by the previous proton irradiation is compensated by a peak of animpurity concentration distribution formed by the second or subsequentproton irradiation.
 5. The method for producing a semiconductor deviceaccording to claim 1, wherein, in the proton irradiation step, a depthfrom the other main surface of the n-type semiconductor substrate atwhich the reduction in mobility due to the disorder is the maximum afterthe previous proton irradiation is the irradiation depth in the nextproton irradiation.
 6. The method for producing a semiconductor deviceaccording to claim 1, wherein, when the common logarithm log(E) of theacceleration energy E of the proton irradiation is y and the commonlogarithm log(Rp) of an average range Rp of the proton irradiation fromthe other main surface is x, y =−0.0047x⁴+0.0528x³−0.2211x²+0.9923x+5.0474 is satisfied.
 7. The method for producing a semiconductor deviceaccording to claim 1, wherein the semiconductor device is a diode or anIGBT.
 8. The method for producing a semiconductor device according toclaim 1, wherein an acceleration energy of proton irradiation is a valuethat a projected range of proton irradiation does not exceed thethickness of the n-type semiconductor substrate.